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Functional control at the nanoscale forms the foundation of biological systems. These work at the cellular level by manipulating ions and molecules. Nanoscale devices that give functional control at this scale are also becoming an important component in diverse fields such as electronics, medicine, engineering, and manufacturing. Moreover, scientific advances are making it technologically viable to control and observe matter at the atomic level. Computational techniques are an integral part in designing and understanding these systems, as they reveal the processes that are experimentally unobservable and allow for inexpensive screening and predictions. Since the atomistic nature of matter plays a major role at the nanoscale, classical all-atom molecular dynamic simulations are widely employed.
We concentrate on thermal transport, ion transport, and nonlinear interactions at the nanoscale. For thermal transport, in particular, we develop a rigorous approach to computing thermal conductance by coupling a system of interest to two large ``extended reservoirs that act as heat source/sinks. Within this setup, we prove that the ``intrinsic conductance of the system can be obtained by having the extended reservoirs be large and weakly coupled to Langevin baths.
For ion transport, we show that subnanoscale pores in monolayer graphene membranes display selectivity (not unlike biological ion channels but weaker) even when the graphene pores do not have charge or functional groups. The selectivity appears because the ions translocating through these pores lose some water from their solvation shell and different ions have different energy dehydration penalties. We also demonstrate that such selectivity can be tuned by adjusting the pore radius and number of graphene layers. This will enable the optimization of water flow and ion rejection for applications such as filtration and desalination. We also develop a finite-size scaling model to compute the effect of bulk electrolyte dimension on the ionic resistance. This separates the pore and the bulk electrolyte contribution to the total ionic resistance in molecular dynamics simulations. Additionally, in collaboration with an experimental group, we show that graphene is a good choice for a transparent cap for spectroscopy of water as it only has a minor influence on the structure of water.